Blocking microleaks in flexible food packages

ABSTRACT

Microleaks in flexible packages constituting an avenue for microbiological contamination of the contents are blocked by immersion of the package in an elastomer polymer solution having a viscosity of 10 to 1000 centipoises which solution enters into the microleak and upon removal of the solvent deposits an elastomer plug which completely and effectively closes the microleak.

United States Patent 1191 Killoran et a1.

[ BLOCKING MICROLEAKS IN FLEXIBLE FOOD PACKAGES [75] lnventors: John J. Killoran, Worcester; Frank J. Rubinate, Needham, both of Mass.

[73] Assignee: The United States of America as represented by the Secretary of the Army, Washington, D.C.

221 Filed: July 22, 1971 21 Appl. No.: 165,387

[52'] US. Cl. 99/171 LP, 117/94, l17/138.8 F,

[ Oct. 16, 1973 3,580,464 5/1971 Griffith 220/67 X 3,068,106 12/1962 Hall 99/170 UX 3,552,998 1/1971 Weyna et a1. 117/76 F 2,787,552 4/1957 Seiferth 99/174 3,528,826 9/1970 Wilson 99/171 R UX 2,876,110 3/1959 Stadelman et a1. 99/170 1,324,662 12/1919 Goldsworthy 99/77.1 3,644,571 2/1972 Anderson et a1. 161/214 X FOREIGN PATENTS OR APPLICATIONS 552,310 l/l958 Canada 99/171 CA Primary Examiner-Frank W. Lutter Assistant Examiner-Steven L. Weinstein Att0rney1-1arry M. Saragovitz et a1.

[5 7] ABSTRACT Microleaks in flexible packages constituting an avenue 'for microbiological contamination of the contents are blocked by immersion of the package in an elastomer polymer solution having a viscosity of 10 to 1000 centipoises which solution enters into the microleak and upon removal of the solvent deposits an elastomer plug which completely and effectively closes the microleak.

7 Claims, No Drawings BLOCKING MICROLEAKS'IN FLEXIBLE FOOD PACKAGES BACKGROUND OF THE INVENTION This invention relates to a method of effectively blocking microleaks in food-filled, flexible packages to prevent microbiological contamination of the microbiologically-stable contents. 5

Thermally-processed foods have historically been packaged in rigid containers of metal or glass and a high confidence level has been established with respect to the ability of such rigid packages to maintain the microbiological integrity of the contents. With advances in packaging technology involving the development of special flexible packaging materials, it has become technically feasible to package thermally processed or other microbiologically stabilized foods in flexible, laminate, packaging materials. A high confidence level has not, however, been established for such flexible packaging materials since package failure rates of approximately 0.3 percent have been repeatedly demonstrated when such materials have been used to package thermally processed foods. It will be necessary to achieve package failure rates no greater than 0.01 percent before such packaging can be considered practical for thermo-processed foods. In this context, package failure refers to some defect which provides an avenue for bacterial contamination of the package contents.

Defects exist either in the body of the package or in the seal area, and typically consist of wrinkles or holes in the seal, or pinholes in the body. Under simulated conditions, it has been demonstrated that bacterial contamination can occur through pinholes or openings at least as small as 33 microns in diameter. At the present time, there is no rapid non-destructive test that would determine the existence of microscopic pinholes (less than 160 microns) in a flexible package. Without such a test, it would be virtually impossible to further reduce the package failure rate by elimination of defective packages. We have, however, discovered a method which substantially reduces the package failure rate, not by detection and elimination of packages containing microscopic leaks but by employing a technique which blocks existing microleaks ranging in size from approximately microns to 300 microns, thereby maintaining the integrity of the package and its contents.

SUMMARY OF THE INVENTION Elimination of microleaks or other openings ranging in size from 10 microns to 300 microns in flexible packages containing microbiologically stable food stuffs is accomplished by covering the sealed package with a solution of an elastomer polymer which coats the package, penetrates into and plugs the microleaks. The polymer solution passes into the microleak and upon subsequent removal of the solvent, the polymer forms a hard plug that tenaciously adheres to the opening. The polymer employed is a low molecular weight, block copolymer of styrene and butadiene. The polymer is completely dissolved in an organic solvent in an amount sufficient to provide a solution viscosity of from about 10 to about 1,000 centipoises.

DESCRIPTION OF THE PREFERRED EMBODIMENT Flexible package defects are difficult to detect due to which areof limited use as inspection sampling techniques. In a bio-test method, a flexible package containing thermally-processed food is immersed in and flexed in a bath containing Aerobacter aerogenes at a concentration of 6 X 10 viable cells per ml. and subsequently incubated at 38 C. for 48 hours. In packages having pinholes at least as small as 33 microns, contamination of the contents is observed using this test method. Further details on the bio-test method and ap! paratus are described in an article by Maunder, Folinazzo and Killoran, appearing in Food Technology, Volume 22, No. 5, Pages 81 to 84 (1968). A second test method is an internal, pressure-under water-test which requires that the flexible package be submerged in water and be pressurized with nitrogen gas by inserting nitrogen into the package via a fine hollow needle. After the package expands and the internal pressure builds up, the escape of gas bubbles will indicate the presence of any small openings. While both methods will determine the presence of microleaks, neither is suitable as a production test since they are both basically destructive tests.

The flexible packaging materials which may be used in connection with the present invention include flexible laminates conventionally used for food packaging. Preferred are those laminates having as an outer ply or layer, polyethylene terephthalate or polyiminocaproyl films. Aluminum foil, an excellent gas barrier, is conventionally employed as an intermediate layer, and a heat scalable polymer, such as polyethylene, is employed as the inner layer. Packages constructed of such materials may contain defects which defects can result from the manufacture of the laminate itself, or be produced as a consequence of the form-fill-and seal operation, or arise during the processing step which renders the contents of the package microbiologically stable. Pinholes in the body of the package and holes or wrinkles in the closure seal are the principal defects. While packages having openings or leaks, more than microns in diameter, could be visually detected and removed, smaller defects, less than 160 microns, cannot be visually detected and, consequently, cannot be readily eliminated.

In accordance with the present invention, each sealed package is covered with an elastomer polymer solution, which penetrates into any microleak present in the package. After evaporation of the solvent, the polymer forms a tenacious plug in the microleak which will prevent microbial access to the interior of the package. The elastomer polymer used in the present invention is a block copolymer of styrene and butadiene, a highly soluble, thermoplastic material. These polymers have the ready solubility of unvulcanized polymers and the stress-strain properties of conventional vulcanizates. Such polymers are commercially available and are soluble in wide range of organic solvents, such as toluene, methylethylketone, diethyl ether, bromobenzene, cyclohexane, benzene, etc. While excess of 30 percent by weight of the polymer may be dissolved in a suitable solvent, it is essential for purposes of this invention that the treating solution contain by weight sufficient polymer to provide a solution viscosity ranging from about 10 to about 1,000 centipoises. The preferred solvent system for purposes of this invention is a blend of 4 parts by weight of methylethylketone and 1 part by weight of toluene. In such a system, the polymer solids content ranges from 5 percent by weight to 15 percent by weight. Solutions of polymers of the type described, having viscosities ranging from to 1,000 centipoises will readily enter microleaks, i.e., openings into the interior of the package ranging in size from 10 microns to 300 microns, but not penetrate in any substantial quantity to the interior of the package.

The polymer solution may be brushed or sprayed on the package, but for rapid application, it is preferred to totally immerse the package in the solution. Application of the polymer solution is made at ambient room temperatures, i.e., -30 C. Solvent removal is effected by air drying at ambient temperatures or by forced drying in an air circulating chamber at temperatures up to 70 C. The following Examples describe in detail specific embodiments of this invention.

EXAMPLE 1 A test pouch was formed of a laminate material comprising from the outer layer to the inner layer of 1.0 mil polyiminocaproyl, 0.35 mil of aluminum foil and a 2.0 mil layer of a chemically bonded polyethylene terephthalate and medium density polyethylene. The pouch was filled'with a plywood block and the pouch opening closed by heat sealing the opposed polyethylene surfaces together. Defects were created in the body of the pouch by piercing the pouch with a series of fine needles to obtain holes ranging in size from 35 microns to 160 microns. A polymer solution was prepared by dissolving a commercially available block copolymer of styrene and butadiene in a mixture of four partsby weight of methylethylketone and one part by weight of toluene. The particular polymer employed is Kraton 1101, a product of Shell Chemical Company, New York, New York, having a specific gravity of 0.94. Typical physical properties of the polymer at 23 C. include a tensile strength of 4,600 psi, a 300 percent modulus of 400 psi, and an elongation of 880 percent. The solution contains 1 1 percent by weight of the polymer. Six packages, as described, each having four separate holes in the body area, were dip coated in the solution at room temperature. The withdrawal rate from the solution was 4 inches per minute. The coated packages were air dried for 60 minutes. The pouches were individually submerged in water and pressurized internally with nitrogen gas introduced through a hollow needle penetrating the pouch. The internal pressure within the package was raised to approximately 6 pounds per square inch. No gas bubbles were detected escaping from any of the pouches indicating that the openings had become completely and tightly sealed. The depth of penetration of polymer material into the hole was determined by adding a dye, Rhodamine B, to the polymer solution and examining the holes visuallyunder magnification. in the case of holes in the 50 micron to 160 micron range, it was observed that the polymer had penetrated approximately 50 percent of the way through the hole.

EXAMPLE ll Pouches were prepared by heat sealing together the I edges ofa laminate material consisting of an outer layer of 0.5 mil polyethylene terephthalate, a middle layer of 0.35 mil aluminum foil and an inner 3.0 mil layer of high density polyethylene. The package pouch was filled with chicken a la king, the pouch sealed and the package and its contents sterilized with high energy ionizing radiation from a Cobalt 60 source. A sterile No.- 14 sewing needle was used to separately pierce each of 50 sterile packages. Packages so pierced, had microleaks ranging in size from approximately 20 microns to approximately microns. These packages were coated with the polymer solution of EXAMPLE I modified to contain 15 percent by weight of the polymer. Each of the 50 packages are separately immersed in a solution in a dip coating operation with the speed of withdrawal being 3 inches per minute. The packages were allowed to air dry for 1 hour. The pierced and coated packages were flexed in the Aerobacter aerogenes solution in a bio-test unit for 15 minutes and subsequently incubated for 48 hours. Fifty of the same sterilized pouches pierced, but not coated, were similarly tested as were 50 sterilized but not pierced control packages. The pierced and coated packages and the control packages failed to show any swelling after 48 hours indicating that no microbial contamination had occurred. The fifty pierced, but uncoated packages were all swollen at the end of the 48 hour incubation periodindicating that microbial contamination had occurred through the microleaks in the packages.

Microleaks in packages ranging in size from approximately 10 microns up to 300 microns are satisfactorily sealed and blocked using the elastomer polymer solution described herein.

Since packages having defects or leaks greater than 160 microns. can be visually detected and readily screened out, then treatment of all remaining packages, as described herein, is a procedure that will significantly reduce the failure rate of flexible packages to a level approaching that of rigid containers.

We claim:

1. A method of blocking microleaks in a flexible, laminated sealed package having microbiologicallystable contents therein, said package having microleaks on the order of 10-300 microns in size which method comprises (a) applying a solution of block copolymer of styrene and butadiene, having a solution viscosity of from 10 to 1,000 centipoises over the exterior of the flexible, sealed package to completely cover all of the external surfaces of said package, and (b) removing the solvent from the solution covered package causing the polymer which penetrated within the microleaks to form an elastomer plug which prevents microbiological access to the interior of the package.

2. A method according to claim 1 wherein said solution is present in an amount of from 5 percent to 15 percent by weight based on the weight of the solution.

3. A method according to claim 2 wherein the solvent for said solution is a mixture of 4 parts by weight of methylethylketone and 1 part by weight of toluene.

4. A method according to claim 3 wherein the microbiologically-stable contents of said package is a food product.

5. A method according to claim 4 wherein said polymer solution contains about 11 percent by weight of said polymer.

6. A method according to claim 5 wherein the outer ply of said flexible package is polyethylene terephthalate.

7. A method according to claim 5 wherein the outer ply of said flexible package is polyiminocaproyl.

i i i 

2. A method according to claim 1 wherein said solution is present in an amount of from 5 percent to 15 percent by weight based on the weight of the solution.
 3. A method according to claim 2 wherein the solvent for said solution is a mixture of 4 parts by weight of methylethylketone and 1 part by weight of toluene.
 4. A method according to claim 3 wherein the microbiologically-stable contents of said package is a food product.
 5. A method according to claim 4 wherein said polymer solution contains about 11 percent by weight of said polymer.
 6. A method according to claim 5 wherein the outer ply of said flexible package is polyethylene terephthalate.
 7. A method according to claim 5 wherein the outer ply of said flexible package is polyiminocaproyl. 